1. Field of the Invention
The present invention generally relates to mobile communication systems, and more particularly to a system and method for performing automatic frequency control in a mobile communication terminal.
2. Background of the Related Art
In a mobile communication system, frequencies must be adjusted to be the same between comnunication-connected objects. This is especially true in cellular communication systems where terminals operate in synchronization with a base station. In a CDMA (Code Division Multiple Access) system, for example, signals are spread using a specific spread code and then transmitted. At the receiver side, the signals are reverse spread and demodulated to their original states.
In communicating signals in this manner, phase synchronization is performed between the spread codes of a transmitting side and receiving side. Without this phase synchronization, the original signal cannot be demodulated. In synchronizing the frequency of the mobile terminal to that of the base station, an automatic frequency controlling (AFC) is performed.
A related-art automatic frequency controller includes a frequency displacement detector and a crystal oscillator, which has capability of performing fine controlling based on an analog voltage. The frequency displacement detector extracts a sinusoidal signal component from a demodulated signal, measures the tilt of its phase, and then calculates a frequency difference between the transmitting side and receiving side based on the tilt. The crystal oscillator is controlled based on the calculated frequency difference, and a phase of a signal received afterwards is adjusted to be close to a horizon.
In mobile communication systems of the aforementioned type, frequencies are corrected according to fine changes in the transmitting side, various paging environments and movements of the receiving side. As these effects vary, the frequency displacement detector must keep pace.
The related-art method for detecting frequency displacement has a number of drawbacks. For example, because frequencies are affected by noise generated during transmission and from the periphery of the mobile communication terminal, it is difficult to detect an accurate frequency correction value, and also additional controlling is required to minimize the influence of noise. This complicates the process of performing frequency control and therefore makes the related-art method undesirable.